[Snyk] Security upgrade golang from 1.20.5 to 1.21rc3

[xcaspar]

This PR was automatically created by Snyk using the credentials of a real user.

Keeping your Docker base image up-to-date means you’ll benefit from security fixes in the latest version of your chosen image.

Changes included in this PR

• build/image/musl/Dockerfile

We recommend upgrading to golang:1.21rc3, as this image has only 69 known vulnerabilities. To do this, merge this pull request, then verify your application still works as expected.

Some of the most important vulnerabilities in your base image include:

Severity	Priority Score / 1000	Issue	Exploit Maturity
	221	CVE-2021-32256 SNYK-DEBIAN12-BINUTILS-5803143	No Known Exploit
	221	CVE-2021-32256 SNYK-DEBIAN12-BINUTILS-5803143	No Known Exploit
	221	CVE-2021-32256 SNYK-DEBIAN12-BINUTILS-5803143	No Known Exploit
	436	CVE-2023-32001 SNYK-DEBIAN12-CURL-5783177	No Known Exploit
	321	Inappropriate Encoding for Output Context SNYK-DEBIAN12-OPENSSH-1556053	Mature

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